Traditionally the popper valves of an engine have been actuated by one or more camshafts which are mechanically driven from the engine crankshaft at half the engine speed, thereby operating the valves in synchronism with engine rotation, and in a fixed phase with one another. It is also known to substitute rotary valves for popper valves, again mechanically driving the valves from the crankshaft and rigidly slaving the valve operation to engine rotation.
It is known that the performance of engines can be improved by variable valve timing since the optimum timing is dependent on speed and load conditions. To change valve timing, it has been proposed to mechanically adjust the camshaft angle, in some cases using an electric motor to make the adjustment.
It is also known that engine performance can be further enhanced by controlling not only engine-valve timing, but also other aspects of valve operation such as the duration of open periods. To that effect, various mechanisms have been proposed such as direct, independent valve actuators moved by pneumatic, hydraulic or electromagnetic forces. While providing valve-profile flexibility, such mechanisms have often suffered various problems such as: inadequate control of the valve seating velocity, high energy consumption, and relatively long response time that precludes high engine speed operation. It is therefore advantageous to provide means of operating engine valves that give the desired high degree of valve-profile flexibility and at the same time feature the necessary low valve-seating velocity, allow the engine to operate over a standard speed range and have low energy requirements.